1. Field of the Invention
This invention relates to a member of a hot dip galvanizing line used in contact with a hot dip galvanizing bath in the process of producing hot dip galvanized steel sheet and to a process for producing the member.
2. Description of the Background Art
In a production line for plating the surface of steel sheet with zinc or zinc alloy by reduction-treating cold-rolled steel sheet in a heat-treatment furnace and then immersing it in a hot dip galvanizing bath, the practice has been to fabricate the members of the hot dip galvanizing line making contact with the hot dip galvanizing bath of iron or ceramic, or of stainless steel or other metal. Among these materials, ceramic is excellent in erosion resistance but is brittle, while metallic materials such iron and stainless steel are excellent in strength and other mechanical properties but are readily eroded by the hot dip galvanizing bath.
This is a particular problem in the case of the sinker roll that supports the steel sheet in the hot dip galvanizing bath and alternately changes its direction of travel between downward and upward as it passes through the bath. The wear and erosion these rolls incur in turn degrades the quality of the hot dip galvanized steel sheet, most notably its surface properties.
Methods for preventing this include that of flame spraying the surface of the sinker roll with a coating of either a self-fluxing alloy containing nickel, cobalt and other metals or tungsten carbide containing cobalt, that of flame spraying them with a coating of an oxide-type ceramic with extremely low wettability, and that of flame spraying them with a film of silicon nitride or Sialon (Japanese Patent Application Laid-open Nos. 4-254571 and 6-228724).
These prior art technologies are not entirely satisfactory, however, owing to the following problems.
The self-fluxing alloy containing cobalt, nickel etc., the tungsten carbide containing cobalt, and other such coatings are degraded by reactions with the zinc in the hot dip galvanizing bath. Since the cobalt and nickel are present in the coating in metallic state, they drop out upon forming alloys with the zinc, leading to eventual disintegration of the coating itself.
Since the coating obtained by flame spraying a ceramic such as oxide or silicon nitride contains many bubbles and cracks, it is low in coating strength and soon peels off. Since the adhesive strength between the flame-sprayed coating and the base metal is weak, moreover, this coating has a number of drawbacks, including poor wear resistance and weak thermal shock resistance.
It is also expensive owing to the complex fabrication process which involves additional nitriding of the coating after flame spraying.
The object of the invention is to overcome the foregoing problems.